Total organic carbon

TOC, Total organic carbon — The amount of carbon bound in organic compounds. Often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment.

A typical analysis for TOC measures both the total carbon (TC) present as well as the inorganic carbon (IC). Subtracting the inorganic carbon from the total carbon yields TOC. Another common variant of TOC analysis involves removing the IC portion first and then measuring the leftover carbon. This method involves purging an acidified sample with carbon-free air or nitrogen prior to measurement, and so is more accurately called non-purgeable organic carbon (NPOC).

Virtually all TOC analyzers measure the CO₂ formed when organic carbon is oxidized and/or when inorganic carbon is acidified. Oxidation is performed either through Pt-catalyzed combustion or with a UV/persulfate reactor. Once the CO₂ is formed, it is measured by a detector: either a conductivity cell (if the CO₂ is aqueous) or a non-dispersive infrared cell (after purging the aqueous CO₂ into the gaseous phase). Conductivity detection is only desirable in the lower TOC ranges in deionized waters, whereas NDIR detection excels in the higher TOC ranges. A variation described as Membrane Conductivity Detection can allow for measurement of TOC across a wide analytical range in both deionized and non-deionized water samples. Modern high-performance TOC instruments are capable of detecting carbon concentrations well below 1 µg/L (1 part per billion or ppb).

TOC is the first chemical analysis to be carried out on potential petroleum source rock. It is very important in detecting contaminants in drinking water, cooling water, water used in semiconductor manufacturing, and water for pharmaceutical use. Analysis may be made either as an online continuous measurement or a lab-based measurement.

TOC detection is an important measurement because of the effects it may have on the environment, human health, and manufacturing processes. TOC is a highly sensitive, non-specific measurement of all organics present in a sample. It can therefore be used to regulate the organic chemical discharge to the environment in a manufacturing plant. Low TOC can also confirm the absence of potentially harmful organic chemicals in water used to make pharmaceuticals. TOC is also of interest in the field of potable water treatment due to disinfection byproducts formed in chlorination and ozonation reactions. Inorganic carbon poses little to no threat.

See also Standard Method 5310, Disinfection by-Product Rule (DBR), US Pharmacopiea Chapter 643, ASTM D5997, ASTM D5904, ASTM D6317.